In the development of plasma-arc technology over the past twenty-five years, equipment improvements have made the transferred-arc torch designs much more reliable than their non-transferred-arc counterparts. This fact is particularly true when operating at high gas pressure, high arc column amperage, or both.
Transferred-arc plasma torches are most commonly used for metal cutting and welding. High reliability results from the anode electrode being exterior to the torch. The arc actually passes to the piece being cut or welded, and that piece or a component thereof functions as the anode in the arc process. The constructing nozzle functions simply as a passageway for the arc column. The additional anode heating is not super-imposed on the constricting nozzle.
In contrast, in the non-transferred-arc torch, often used in flame spraying of metals and ceramics to form a coating, the plasma-directing nozzle must also serve as the anode electrode (assuming straight polarity). These plasma directing nozzles are easily overheated and fail much more frequently than where they are used in conjunction with a transferred-arc. Because of the weakness of the nozzle of the non-transferred design, small nozzle diameters required to produce high jet velocities are not commercially useful. On the other hand, transferred-arc apparatus for cutting metal frequently is designed to produce supersonic jet flows at high current flow.
It was noted that in observing a transferred-arc torch functioning to pierce a hole in a one-half inch thick steel plate, the arc column melts its way through the full thickness of the steel, first producing a small diameter hole. With continued arc heating and plasma scouring, the hole grew in diameter. When it reached about one-half inch diameter, the arc voltage requirement became so high that the power source could no longer provide it and the arc went out.
Based on this observation, it is an object of the present invention to combine the advantages of the transferred-arc torch with a novel anode spaced from or electrically isolated from the torch and its cathode, and spaced from but coaxial with the flow constricting nozzle associated with the transferred-arc torch, to permit the transferred-arc torch to function as a non-transferred-arc torch.